There is an ongoing need for adhesive structures having improved adhesion obtained through physical attractive forces. Such structures can be suited to use in various applications, such as medical applications, e.g., as an adjunct or replacement to sutures and staples used to close surgical incisions. For the adhered to substrate, e.g., living tissue, providing an adhesive structure that provides adhesive forces by non-chemical interactions between adhesive structure and substrate would be highly desirable.
Intermolecular forces are exerted by molecules on each other and affect the macroscopic properties of the material of which the molecules are a part. Such forces may be either attractive or repulsive in nature. They are conveniently divided into two classes: short-range forces, which operate when the centers of the molecules are separated by 3 angstroms or less, and long-range forces, which operate over greater distances.
Generally, if molecules do not interact chemically, the short-range forces between them are repulsive. These forces arise from interactions of the electrons associated with the molecules and are also known as exchange forces. Molecules that interact chemically have attractive exchange forces, also known as valence forces. Mechanical rigidity of molecules and effects such as limited compressibility of matter arise from repulsive exchange forces.
For present purposes, physical attractive forces are considered to be attractive forces that are not chemical in nature, e.g., not dependent on or associated with ionic bonding, covalent bonding, or hydrogen bonding. Physical attractive forces can include long-range forces or van der Waals forces as they are also called. These forces account for a wide range of physical phenomena, such as friction, surface tension (capillary actions), adhesion and cohesion of liquids and solids, viscosity and the discrepancies between the actual behavior of gases and that predicted by the ideal gas law. Typical bond energies from van der Waals forces are about 1 kcal/mol compared to about 6 kcal/mol for hydrogen bonds and about 80 kcal/mol for carbon-to-carbon bonds. Van der Waals forces arise in a number of ways, one being the tendency of electrically polarized molecules to become aligned. Quantum theory indicates also that in some cases the electrostatic fields associated with electrons in neighboring molecules constrain the electrons to move more or less in phase.
The London dispersion force otherwise known as quantum induced instantaneous polarization (one of the three types of van der Waals forces) is caused by instantaneous changes in the dipole of atoms, resulting from the location of the electrons in the atoms' orbitals. When an electron is on one side of the nucleus, this side becomes slightly negative (indicated by δ−); this in turn repels electrons in neighboring atoms, making these regions slightly positive (δ+). This induced dipole causes a brief electrostatic attraction between the two molecules. The electron immediately moves to another point and the electrostatic attraction is broken. London dispersion forces are typically very weak because the attractions are so quickly broken, and the charges involved are so small.
Despite the weakness of van der Waals forces, it has been recognized that such forces can contribute to adhesion by a structure formed in nature. For example, it has been observed that the adhesive force of a gecko's foot is attributable to the van der Waals forces generated by hundreds of thousands of fibrillar, hair-like microstructures known as setae, which terminate in even smaller structures (200 to 400 nanometers in diameter) known as spatulae. Such structure permits a gecko to climb even smooth surfaces such as vertical planes of glass, achieving adhesion without any requirement that the target substrate itself provide adhesive characteristics. Structures mimicking a gecko's foot have been attempted by various methods including nano-molding using a template, polymer self-assembly, lithography, and etching. However, such structures are inherently delicate and can suffer from durability problems in practical applications. Accordingly, structures offering adhesion attributable to van der Waals forces but with simpler shapes and construction are desirable.
U.S. Pat. No. 6,872,439 proposes a fabricated microstructure comprising at least one protrusion capable of providing an adhesive force at a surface of between about 60 and 2,000 nano-Newtons. A stalk supports the protrusion at an oblique angle relative to a supporting surface, and the microstructure can adhere to different surfaces.
U.S. Pat. No. 7,479,318 relates to a fibrillar microstructure and processes for its manufacture. These processes involve micromachining and molding, and can be used to prepare sub-micron dimensioned fibrillar microstructures of any shape from polymeric as well as other materials.
WO 2008/076390 teaches dry adhesives and a method for forming a dry adhesive structure on a substrate by forming a template backing layer of energy sensitive material on the substrate, forming a template layer of energy sensitive material on the template backing layer, exposing the template layer to a predetermined pattern of energy, removing a portion of the template layer exposed to the predetermined pattern of energy, and leaving a template structure formed from energy sensitive material and connected to the substrate through the template backing layer.
WO 2009/067482 proposes an adhesive article that includes a biocompatible and at least partially biodegradable substrate having a surface; and a plurality of protrusions extending from the surface. The protrusions include a biocompatible and at least partially biodegradable material, and have an average height of less than approximately 1,000 micrometers.
A review of the prior art shows use of micro-nano structures on polymer substrates for adhesion to tissue (WO 2009/067482), but the materials used to fabricate these structures comprise “softer” polymers, i.e., polymers or polymer mixtures having a Young's modulus ≦17 MPa. Moreover, they do not provide a solution for adhesion to specific types of tissue.
It would be desirable to provide an adhesive structure without relying solely on surface chemical groups to provide acceptable conformal contact and adhesion with its intended target surface.
It would also be desirable to provide an adhesive structure that has a stiffness (Young's modulus) greater than 17 MPa that provides a means by which a fluid such as the tissue's own fluid or a chemical group such as a fibrin sealant can wick into the structure to enhance adhesion with its intended target surface.